Low carbon steel alloy for vitreous enameling



Nov. 1, 1966 E. H. MAYER ETAL 3,282,685

LOW CARBON STEEL ALLOY FOR VI'I'REOUS ENAMELING Filed Aug. 8, 1963 5 Sheets-Sheet 1 EFFECT OF MOLYBDENUM 0N E TCH RATE MOL YBDENUM INVENTORS Edward H. Mayer BY H/720n Al Rah/7 Evan M Oliver Nov. 1, 1966 E. H. MAYER ETAL 3,282,685

LOW CARBON STEEL ALLOY FOR VITREOUS ENAMELING 3 Sheets-Sheet 2 Filed Aug. 8, 1963 EFFECT OF COPPER 0N ETCH RATE mm Ma w 00 0 u| |H\ I 0 8 m. w w w w m 55F 93 mu E am 55 COPPER INVENTORS Nov. 1, 1966 E. H. MAYER ETAL 3,

LOW CARBON STEEL ALLOY FOR VITREOUS ENAMELING 5 Sheets-Sheet 5 Filed Aug. 8, 1963 EFFECT OF T/N 0N ETCH RATE III.

% T/IV INVENTORS Edward h. Mayer H/fon /V. Hahn Evan M. Oliver United States Patent 3,282,685 LOW CARBON STEEL ALLOY FOR VITREOUS ENAMELING Edward H. Mayer, Hilton N. Rahn, and Evan M. Oliver, Bethlehem, Pa., assignors, by mesne assignments, to Bethlehem Steel Corporation, a corporation of Delaware Filed Aug. 8, 1963, Ser. No. 300,849 2 Claims. (Cl. 75-425) This invention is directed to a very low carbon vitreous enameling steel having a controlled etching rate as a result of a controlled range of residual alloy content.

In recent years so called zero carbon or special low carbon steels of approximately the following composition have been widely adopted as a base for vitreous enamel coatings:

Percent Carbon, maximum .010 Manganese v .l5/.50 Phosphorus, maximum .010 Sulphur, maximum .030

The development of these steels eliminated the source of many of the surface defects in the vitreous enamels, and since it was no longer necessary to use a blue ground coat of enamel to cover up the defects originating in the iron base, efforts to develop single-coat, direct-on enameling practices were greatly accelerated.

In the standard two-coat vitreous enameling practice the ground coat contains sufficient cobalt and/ or nickel to aid in bonding the enamel to the basemetal and the degree of surface roughening of the base metal is not critical. However, wit-h single-coat, direct-on enamels, acid etching of the base metal in cont-rolled amounts and at controlled rates is desirable in order to insure satis factory bond.

Etching should not be confused with pickling. Pickling and etching are two well known, complementary metallurgical :processes that are widely used to condition the surface of ferrous base alloys. Both processes utilize mineral acids, but pickling acid, which is meant to remove accumulated oxide or scale, usually contains an addition agent designed to inhibit the acid attack of the metal proper. In acid etching, where the object is removal of metal, the acid often includes an addition agent designed to accelerate the rate of acid attack on the metal. Although pickling is an extremely complex process it may be considered as a process of using acid to dissolve iron oxide scale whereas in etching the removal of metal is often accompanied by breaking down the acid with the resultant evolution of hydrogen. Technically, pickling is complete when all the scale has been removed. However, etching would immediately commence because even inhibited acid would start to react with the bare metal. -Moreover, since the nature of the oxide scale may vary in different locations it is not unusual for these two complementary processes to take place at the same time. However, in the absence of any oxide film or scale there cannot be any pickling, and etching will be the only result of acid attack.

The proper degree of roughening for enameling is often defined in terms of etching to remove a specific weight of metal for a given unit of area (gms/ sq. ft.) but there has been no general agreement as to the amount of weight loss which is necessary in order to insure enamel adherence adequate to prevent damage to the enamel coating in normal handling and shipping. Heretofore materials from different producers and also different batches of material from the same producer have varied widely in their susceptibility to acid attack. In a porcelain enameling production line this variation in etching characteristics necessitates frequent testing for enamel adherence and modification in the etch tank conditions to compensate for the variation in the base material which in turn has limited the large scale acceptance of direct-on, one-coat enamels in existing enameling lines designed for two-coat enamels. More particularly, it has heretofore been impossible to produce zero carbon enameling irons having uniform and predictable etching rates.

The principal object of this invention is to produce an iron base alloy having sufficient adherence for a direct-on, single-coat vitreous enamel to permit substitution into present two-coat enameling lines without observing any special precautions.

Another object of this invention is to provide an iron base material for single-coat, direct-on vitreous enamels which will have a relatively uniform etching rate and eliminate the necessity for control tests on the enameling line.

Another object of this invention is to provide an enameling iron in which the possibility of over-etching is reduced or eliminated.

Another object of this invention is to provide an enameling iron in which the possibility of under-etching is reduced or eliminated and with it the associated problems due to the lack of an adequate bond for a single-coat, direct-on vitreous enamel coating.

Another object is to provide an iron base material for vitreous enameling which will have a predictable etching rate.

Another object is to provide an enameling iron which will have an etching rate (hereinafter more fully defined) of between .14 and .46 gm/ sq. ft./min.

Another object is to provide an enameling iron which, when flash coated with nickel and thereafter vitreous enameled, according to standard practices for direct-on enameling, will have an adherence index of not less than approximately when subjected to the standard adherence test prescribed by the Porcelain Ename-ling Institute.

Another object of this invention is to produce thin sheets of an iron base alloy having good deep drawing characteristics, a predictable etching rate, resistance to sagging at enamel fir-ing temperatures, freedom from enamel defects and sufficient single-coat directbn enamel adherence to permit normal handling and shipping of the enameled parts without special precautions to avoid failures of the enameled surface.

Although it has been previously recognized that enamel adherence is related to the amount of metal remove-d by acid etching, the effect of etch rate on said amount is a novel discovery of applicants.

These objects are accomplished in the present invention by maintaining the copper, tin and molybdenum contents of a so-called Zero carbon enameling iron within certain specifically defined limits, as is more fully set out hereafter.

In the drawings:

FIG. 1 is a graph wherein the dashed lines depict the extent of the etfect of molybdenum on etch rate.

FIG. 2 is a graph wherein the dashed lines depict the extent of the effect of copper on etch rate.

FIG. 3 is a graph wherein the dashed lines depict the extent of the effect of tin on etch rate.

This invention lies in the compositions of an iron base alloy having a range of etching rates which will result in a weight loss of .14/ .46 gm./ sq. ft./rnin. when immersed in dilute sulphuric acid (containing 6.5% H 80 by weight) at a temperature of 158 F. The iron base alloy of this invention when etched at the above rate to remove a minimum of .75 gm./ sq. ft., flash coated with nickel,

and direct-on, single-coat enameled, Will have a minimum adherence index of not less than approximately 80 when tested by the standard test of the P.E-.I. The P.E.I. adherence test is the standard quantitative test for measuring the adherence of vitreous enamel and was developed at. the Bureau of Standards. This test involves the systematic deformation of the enameled sheet intoa circular die by means of a sphere or rounded plunger. After a suitable interval, adherence is measured electrically by means of a standard series of probes. Experience has shown that an 80 minimum P.E.I. adherence index is sufficient to prevent damaging the enamel coating during normal handling and shipping on a production basis.

It has been hitherto impossible to consistently produce zero carbon enameling sheet steels having the desired etch rate and enamel adherence referred to above. We have developed compositions which will consistently have all of said properties. Our invention consists in (a) producinga steel consisting essentially of following composi tions by weight:

Percent Carbon, maximum .010 Manganese l5 .75 Molybdenum, maximum -1 .049 Copper .014/ .060 Tin .00l/.030

Iron, balance.

and (b) adjusting the proportions of molybdenum, copper and tin in accordance with the following formula .00489/ percent Cu) 6.605. percent Mo) (.084 log percent Sn) (.1367) :X

where X is a number between .14/.46.

The upper limit of .030% tin is dictated not by etch-- Percent Carbon, maximum .010

Manganese .15/.75

' Molybdenum .001/.005 Copper .014/.040

Tin .001/.005

Compositions falling within the above limits will satisfy the formula and produce a sheet steel having good rolling properties which will etch at a rate of .14/.46 gm./ sq. ft./ min. and have good enamel characteristics but these limits should not be considered as limitations of the invention.

FIGS. 1, 2 and 3 are graphs depicting the effects of molybdenum, copper and tin on etch rate. Although the effect of molybdenum shown on FIG. 1 indicates that this element may 'be entirely eliminated, parallel situations do not exist for copper and tin as shown on FIGS. 2 and 3. As the copper and tin approach zero the etch rate begins to approach infinity and we have found that as the etch rate exceeds approximately .46 gm./sq. ft./ min. difiiculties with the enamel are encountered with increasing frequency. These difliculties are in the nature of:

(1) Excessive pickup of nickel during nickel flashing which tends to reduce enamel adherence, and

(2) Accumulation of an etching smut or deposit which requires an additional rinse and/ or scrubbing to prevent enamel defects or discolorations.

Table I shows examples of the composition of eight different heats of zero carbon enameling steels listed in the order of increasing etch rate. Steels 4 to 7 are representative of this invention, and steels 13 and 8 are illustrative of the composition of steels having etch rates outside the scope of this invention.

TABLE I 0 Mn P M0 Cu Sn Etch Rate . 003 34 009 004 066 005 08 004 38 008 015 O32 004 l1 003 32 008 002 050 003 l1 003 .32 008 002 055 003 15 004 32 003 005 018 002 30 004 31 007 006 015 004 35 002 38 014 002 014 002 43 002 34 014 002 013 001 49 The other elements in this iron base alloy are present in the normal amounts expected in steel sheet intended for enameling and following is a specific example of a steel sheet embodying the invention:

Percent Carbon .003 Manganese .35 Phosphorus .01 Sulphur a .026 Silicon .01 Nickel .04

Chromium .02

Aluminum .005 Nitrogen .0025 Copper .03 3 Molybdenum .002 Tin Q. a .004

Iron, balance.

Preferably, we produce this material by making a heat of rimmed steel having a carbon content normal for rimmed steels, e.g. .O6/.08% and manganese .15/.75%. We select the raw materials to keep the molybdenum, copper and tin within the limits expressed by the equation and use a clean furnace to avoid picking up these alloys from the refractories. The steel is poured, rolled, decarburized, etc. in the usual manner to produce a soft material free of organic contaminants and iron oxide.

It is well known that the same ferrous material may vary in its susceptibility to etching in different mineral acids (such as nitric, sulphuric, hydrochloric, etc.) as well as in different concentrations of the same acid. It is also known that additives, such as oxidizing agents, or mixtures of acids affect the reaction rate of a particular etchant. The preferredetchant used in this invention is dilute sulphuric acid because of its low cost, its high relative stability, and the degree to which its rate of reactivity can be modified by changing the temperature. The etchant preferred in the practice of this invention is sulfuric acid in a concentration of 6.5% H by weight in water, and a temperature of 158 F. It should be recognized that although these are the preferred conditions they are not limitations of the invention.

Although the specific example above is a rimmed steel the invention is applicable to. killed and semi-killed steels.

We claim:

1. An iron base alloy containing:

by weight, the molybdenum, copper and tin in said alloy I being in the relationship expressed by the equation .00489/ percent Cu) (6.605 X percent Mo) '-(.084 log percent Sn)(.l367)=X Where X is a number between .14 and .46, the balance of said alloy consisting essentially of iron.

2. An iron base alloy being comprised of:

Percent Carbon, maximum .010 Manganese .15/ .75 Molybdenum .001/ .005 Copper .O14/.040 Tin .001/.005

References Cited by the Examiner UNITED STATES PATENTS DAVID L. RECK, Primary Examiner.

by weight, the balance of said alloy being essentially iron. 10 WETNSTEIN, Assistant Examiner- 

1. AN IRON BASE ALLOY CONTAINING: 